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Autoconf Macro: ax_c_float_words_bigendian
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<h1>
ax_c_float_words_bigendian
</h1>
<h2>
Synopsis
</h2>
<p class="indent" style="white-space:nowrap;">
<code>AX_C_FLOAT_WORDS_BIGENDIAN([ACTION-IF-TRUE], [ACTION-IF-FALSE],
[ACTION-IF-UNKNOWN])</code>
</p>
<h2>
Description
</h2>
<div class="indent">
<p>
Checks the ordering of words within a multi-word float. This check is
necessary because on some systems (e.g. certain ARM systems), the float
word ordering can be different from the byte ordering. In a multi-word
float context, "big-endian" implies that the word containing the sign bit
is found in the memory location with the lowest address. This implemenation
was inspired by the AC_C_BIGENDIAN macro in autoconf.
</p>
<p>
The endianess is detected by first compiling C code that contains a special
double float value, then grepping the resulting object file for certain
strings of ASCII values. The double is specially crafted to have a binary
representation that corresponds with a simple string. In this
implementation, the string "noonsees" was selected because the individual
word values ("noon" and "sees") are palindromes, thus making this test
byte-order agnostic. If grep finds the string "noonsees" in the object
file, the target platform stores float words in big-endian order. If grep
finds "seesnoon", float words are in little-endian order. If neither value
is found, the user is instructed to specify the ordering.
</p>
</div>
<h2>
Author
</h2>
<p class="indent">
Daniel Amelang <dan@amelang.net>
</p>
<h2>
Last Modified
</h2>
<p class="indent">
2006-11-02
</p>
<h2>
M4 Source Code
</h2>
<div class="indent">
<pre class="m4source">
AC_DEFUN([AX_C_FLOAT_WORDS_BIGENDIAN],
[AC_CACHE_CHECK(whether float word ordering is bigendian,
ax_cv_c_float_words_bigendian, [
ax_cv_c_float_words_bigendian=unknown
AC_COMPILE_IFELSE([AC_LANG_SOURCE([[
double d = 90904234967036810337470478905505011476211692735615632014797120844053488865816695273723469097858056257517020191247487429516932130503560650002327564517570778480236724525140520121371739201496540132640109977779420565776568942592.0;
]])], [
if grep noonsees conftest.$ac_objext >/dev/null ; then
ax_cv_c_float_words_bigendian=yes
fi
if grep seesnoon conftest.$ac_objext >/dev/null ; then
if test "$ax_cv_c_float_words_bigendian" = unknown; then
ax_cv_c_float_words_bigendian=no
else
ax_cv_c_float_words_bigendian=unknown
fi
fi
])])
case $ax_cv_c_float_words_bigendian in
yes)
m4_default([$1],
[AC_DEFINE([FLOAT_WORDS_BIGENDIAN], 1,
[Define to 1 if your system stores words within floats
with the most significant word first])]) ;;
no)
$2 ;;
*)
m4_default([$3],
[AC_MSG_ERROR([
Unknown float word ordering. You need to manually preset
ax_cv_c_float_words_bigendian=no (or yes) according to your system.
])]) ;;
esac
])# AX_C_FLOAT_WORDS_BIGENDIAN
</pre>
</div>
<h2>
License
</h2>
<div class="indent">
<span style="white-space:nowrap;">Copyright © 2006 Daniel Amelang
<dan@amelang.net></span>
<p>
Copying and distribution of this file, with or without modification, are
permitted in any medium without royalty provided the copyright notice and
this notice are preserved.
</p>
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